Although it is well known that radiation such as gamma ray (γ-ray), X-ray or the like (hereinafter, which will be simply referred as “gamma ray”) makes large influence on human body, humans cannot see radiation. For example, it is important for operators working in a nuclear electricity generation plant or citizens living in a neighborhood of the nuclear electricity generation plant to reduce radiation exposure.
An apparatus which has portability and is used at a site or the like, the apparatus for determining a radiation source is disclosed in patent documents 1 and 2. The patent document 1 discloses a gamma ray radioactivity distribution imaging apparatus which includes arranged gamma ray detectors and a collimator. The patent document 2 discloses an independent type gamma ray camera which is a portable and self-powered type gamma ray camera for using during an operation.